In the conventional technology, either a control valve driven by a throttle cable or an electromagnetic valve is used to control a pressure in a hydraulic line to be applied to a plurality of friction elements of an automatic transmission. However, configuring the throttle cable between the control valve and a throttle pedal tends to be complicated. Alternatively, a specially designed electromagnetic valve is required, which is not desirable.
Japanese Patent Laid-Open No. 2003-517536 discloses a method for driving a line pressure control valve. The method includes an application pressure control valve controlling a pressure applied to a friction element according to the output pressure of an electromagnetic valve. This method can eliminate the need for a cable between the throttle pedal and the control valve and eliminate the need for a specially designed electromagnetic valve.
However, in the method disclosed in Japanese Patent Laid-Open No. 2003-517536, the pressure applied to the friction element changes according to a change in the output pressure of the electromagnetic valve. Furthermore, the line pressure changes according to the change in the applied pressure. Therefore, the time between initiating a change in the output pressure of the electromagnetic valve and initiating a change in the line pressure is elongated, thereby decreasing the responsivity of controlling the line pressure.